A quantitative understanding of the effect of fluid flow on the microstructure of cast alloys is still lacking. The application of time dependent magnetic fields during solidification offers the possibility to create defined flow conditions in solidification processing. The effect of rotating magnetic fields (RMF) on the microstructure formation in cast Al-alloys (Al-7wt.%Si, Al-7wt.%Si- 0.6wt.Mg) is studied experimentally and numerically. The forced fluid flow conditions result in pronounced macrosegregation effects and affect microstructural parameters. With increasing fluid flow the primary dendrite spacing decreases whereas the secondary dendrite arm spacing increases. The experimental analysis is supported by a rigorous application of numerical modeling with the software package CrysVUn.